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Octoprint plugin for tracing lines and generating toolpaths

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# OctoPrint Tracer

## Introduction
This plugin intends to enable direct generation of toolpaths from physical line 
following. This is illustrated in this video:  
[![Line Follower Octoprint Plugin](https://img.youtube.com/vi/u80e94eu4s0/0.jpg)](https://www.youtube.com/watch?v=u80e94eu4s0)

This could be useful in cases where the object is not originating from a CAD drawing, 
but is coming from a physical object, like cutting a piece to fit into an existing 
odd-shaped hole.

## Installation
Unfortunately, this plugin cannot be installed onto OctoPi directly with automatic 
resolution of the package dependencies.  More detail is provided below, but for the
short version, *before attempting installation* of the plugin, log into the Pi via
ssh and perform these four commands:
```
sudo apt-get update
sudo apt-get install libhdf5-103 libgtk-3-0
source ~/oprint/bin/activate
pip install numpy==1.21.4 opencv-contrib-python==4.5.5.62
```

This has only been tested on OctoPi 0.18.0.  It might work on other versions of 
OctoPrint, and it might work on other platforms like Windows or x86 Linux, but it 
has not been tested and there are no guarantees.

### Side note about wheels
A "wheel" is a precompiled binary Python package that can be installed without having
to recompile locally.  This is important on the Raspberry Pi because attempting 
to compile some packages can take a long time, or fail, or take a long time (many 
hours) *and then* fail.

Wheels are version dependent, and some newer versions of a package may be unavailable
as wheels.  This means that everything might be "fine" at one point in time when 
the current version is available as a wheel, and later, the newer current version may 
be  unavailable as a wheel, and a simple `pip install numpy` might utterly fail
where it used to succeed.

As you can see [here](https://www.piwheels.org/project/numpy/) and
[here](https://www.piwheels.org/project/opencv-contrib-python/), on Python 3.7, some 
of the later versions are unavailable as precompiled binaries.  Standard package
dependency resolution will download the latest source version and compilation will 
fail.

### Manually installing dependencies
By manually installing a particular version, or forcing the latest wheel-only version,
the dependencies can be met and the plugin can install.

To do this, connect via ssh and perform these commands:
```
source ~/oprint/bin/activate
pip install numpy==1.21.4
pip install opencv-contrib-python==4.5.5.62
```

The first line activates the octoprint python virtual environment, and the other two 
lines install versions that are available in binary form according to piwheels.org.

As an alternative, you could do this instead
```
source ~/oprint/bin/activate
pip install --only-binary :all: numpy opencv-contrib-python
```

The second line forces pip to only get wheels.

In addition, opencv requires some libraries that are not present by default, (at 
least on OctoPi), so these two commands are also required:
```
sudo apt-get update
sudo apt-get install libhdf5-103 libgtk-3-0
```

Once the libraries and python packages have been installed, check that numpy and opencv
are available:
```
source ~/oprint/bin/activate
python
```
    
And then to confirm they are working, within the python interpreter
```
Python 3.7.3 (default, Jul 25 2020, 13:03:44)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> np.__version__
'1.21.4'
>>> import cv2
>>> cv2.__version__
'4.5.5'
```

With numpy and opencv properly installed and working, plugin installation should go 
smoothly.

## Preparation
The scan requires a physical printout of the "board96.png" image.  This should be
printed at 96 DPI.  The target should be cut so the paper extends about 2mm 
beyond the head of the arrow.  The arrow should be placed so that the line is about 
10mm in front of the head of the arrow.

The line follower requires that the line to be traced is a single closed curve with no 
intersections (i.e. no figure 8) and with no gaps in the line.  Currently the
line follower is not robust to different line widths or colors, so trial and 
error might be necessary to find a line width that works well.  The line follower 
is also not robust to background clutter (e.g. wood grain) so for best odds of success,
the workpiece should be a relatively uniform, light color.

Manually jog the tool so that the arrow head is near the center of the field of view.

## Configuration
Within the OctoPrint settings, there are options for Camera Tracer.

### Image URL
The image URL is what the server uses to fetch the image.  It is "from the point of view
of the server", so localhost tells it to fetch the image from itself.  The default URL is
"http://localhost/webcam/?action=snapshot" which on OctoPi is the webcam server.

This could be a URL for another server if for example a second Rapsberry Pi or some other 
server were providing images.

### Scan Delay
In some circumstances an image capture may be slightly delayed relative to the physical
movement.  If the image is captured immediately after completion of a "G1" movement 
command, the image may be blurry or in the wrong position.  A delay between the movement 
and the image capture slows down the scanning, but ensures the movement is finished.

### Tool Offset X and Y
This is the relative X and Y position of the center of the camera image and the center of
the spindle.  These represent the movement necessary to move the cutting bit to the 
location where the camera is looking.  Positive numbers would mean the camera location is
at a +X and +Y position relative to the spindle.

### Tool Diameter and Offset
Should be self-explanatory.  The scanning process attempts to detect the center 
of the line, and the toolpath is offset by half the tool diameter, so the edge of 
the cut will fall on the center of the line.

The offset specifies additional "stock to leave", so positive numbers will make holes 
a bit smaller and parts will be a bit bigger.  Negative numbers are permitted, which makes 
holes larger and parts smaller.

### Depth
Currently, the generator does not have a concept of a clearance plane.  The generated
gcode moves to the starting X/Y location, plunges to a Z coordinate of -Depth, cuts the
curve, and then raises to Z=0.  If Z=0 is the top of the workpiece then this will drag the
tool across the top of the workpiece before cutting.

One way to prepare this is to choose a value slightly greater than the workpiece thickness,
touch off the spoilboard, and set the height with G92 to be Z at the negative depth.  For 
example, if the workpiece is 5mm, then the cut depth can be set to -7, and touch off the
spoil board and set G92 Z-7.  Then when the job plunges to Z=-7 it will be at the correct 
depth to cut all the way through the material.

### Feedrate, Hole, and Climb Milling
The feedrate should be self-explanatory, and is applied to the X/Y curve cut speed.  The 
plunge feedrate can't currently be specified separately and is 1/5th of the X/Y feed rate.
Note that the feedrate is specified in mm/min.

The hole checkbox controls whether the toolpath is offset inward (for a hole) or 
outward (for a part) so the dimension of the cut is correct.

Climb milling controls whether the cutting path performs climb milling or conventional 
milling.  A climb-milling hole will cut counter-clockwise around the perimeter, while
a conventional-milling hole will cut clockwise.  For parts, these are reversed, so a
climb milling part will be clockwise and a conventional milling part will cut 
counter-clockwise.

## Operation
To begin the scan, first manually jog so that the arrowhead is near the center of the 
camera field of view.  The location does not have to be precise.

On the Camera Tracer tab, click "Start Trace".  This will begin the process, and if all
goes well, it should first move in the +X direction, then +Y, and then move to the line
and move in steps around the curve.  It should follow the curve once around, and then 
stop.  After it stops, refresh the list of uploaded gcode files, and the newly generated
file should appear there.

If at any time during the scan, things are not going properly, you can click "Cancel" 
from the Camera Tracer tab, and it will stop the process.  If the tracing is proceeding 
and suddenly reverses direction, it indicates that the software got confused about which
way is 'forward' and is tracing the line in the wrong direction.  If the tracing makes 
an entire loop and begins on a second lap, it means that it has failed to detect that it
has gotten back to the starting point.  In either of these cases, it would make sense to 
cancel and try again from the beginning.

## Diagnostics
The octoprint log file may contain useful information if something goes wrong during 
processing.  In addition, the captured images may be downloaded in a zip archive by 
going to the url "/plugins/tracer/last_zip".  These may not be very informative on 
their own, but with an offline replay scaffolding, perhaps they could reproduce the 
error and provide some insight for a future fix.

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